1. Field of the Invention
The present invention relates to encrypted-data decrypting apparatuses that decrypt encrypted data, particularly when a program is executed or used, and further to decrypting apparatuses that decrypt encrypted data while realizing security protection, as well as to such decrypting methods.
2. Description of the Related Art
Conventionally, when data and program that have been encrypted for security purposes are decrypted to be reproduced or executed on a computer system, a decryption support program is executed on the computer system; however, in an open computer system whose specification is open to the public, programs can be easily analyzed and altered. Thus, some illegitimate acts are possible such as (i) to alter a decryption support program and (ii) to change a decrypted program, which is not supposed to be referred to or altered, into one that is referable or alterable.
In order to enhance security protection, other methods have been suggested in which, for example, the decryption support program itself is encrypted, and when data decryption is performed, the decryption support program gets decrypted (Japanese Unexamined Patent Application Publication NO. 09-006232), and authenticity of the decryption support program is checked before the decryption process starts (Japanese Unexamined Patent Application Publication NO. 11-039156). In these cases, it is arranged so that alteration of decryption support programs is prevented in order to protect security of the decrypted programs and data.
Even if the decryption process is performed by executing an authentic decryption support program, when the control is taken over with use of an illegitimate interruption and the like after a decrypted program or data has been loaded into a memory in the computer system, the loaded program or data becomes referable or alterable, and the security cannot be protected.